Trona is an ore which contains sodium carbonates and sodium bicarbonates, collectively referred to as sodium sesquicarbonates. In Trona, sodium sesquicarbonates typically are found with clays, shales, and the like. Separation and removal of clays, shales and the like is necessary to provide the sought after sodium sesquicarbonates.
Flocculating clays and shales from Trona typically has been performed in the prior art by either the monohydrate slurry method or the sesquicarbonate method. The monohydrate slurry method entails calcining Trona at 200.degree.-400.degree. F. to convert the sodium sesquicarbonates to sodium carbonates. Lower calcining temperatures may be employed when lower concentrations of sodium sesquicarbonate are present in the Trona. The calcined Trona containing the sodium carbonates is then dissolved in water at 25.degree.-100.degree. C. within a pH range of 9-11, depending on the concentration of sodium sesquicarbonates in the Trona, to provide the monohydrate slurry. The resulting monohydrate slurry is treated with a flocculating agent to flocculate solids such as clays, shales and the like from the slurry to provide a supernatant solution of sodium carbonate. The sodium carbonates then are precipitated from the supernatant solution and heated to yield dry sodium carbonate.
In the sesquicarbonate process, Trona typically is dissolved in water at a temperature of about 200.degree. F. and at a pH of 9-11. A flocculating agent is added to precipitate solids such as clays and shales to yield a clarified supernatant liquid of sodium sesquicarbonate. The supernatant solution of sodium sesquicarbonates is cooled to precipitate sodium sesquicarbonates which are calcined to yield sodium carbonates.
Various flocculating agents have been employed to flocculate clays, shales, etc. from slurries of Trona. Synthetic polymers such as acrylamide polymers typically have been used as flocculating agents. Saccharides such as guar gum also have been used to flocculate clays and shales. Flocculating agents such as guar gum, however, tend to provide low rates of flocculation. Guar gum, moreover, is expensive.
A need therefore exists for an inexpensive method of providing high rates of settling of solids such as clays and shales from mineral slurries such as Trona slurries.
Conventional flocculating agents such as copolymers of acrylamides and acrylates also have been employed to flocculate alkaline earth carbonates from caustic slurries such as slurries containing slaked lime and sodium carbonate. These copolymers, however, tend to produce low rates of precipitation of alkaline earth carbonates. Also, the clarities of supernatant liquids that result from precipitation of the alkaline earth carbonates tend to be low, indicating that substantial quantities of alkaline earth carbonates remain in the supernatant solution. The supernatant liquid that results from flocculation of the alkaline earth carbonates, moreover, often includes coloration impurities such as iron, humic acid and organic materials. Elimination of these coloration impurities is desirable because these impurities reduce the quality of the alkaline earth carbonates precipitated from the supernatant solution.
A need therefore exists for methods to efficiently and effectively precipitate alkaline earth carbonates from caustic solutions. A further need exists for methods of reducing the levels of coloration impurities in supernatant solutions of alkaline earth carbonates.